Chess.el does not know how to play chess against you. While the
library does know all legal moves, there is no "thinking" module. For
this, you must download one of the publically available chess engines,
such as gnuchess, crafty or phalanx. You will find all of these
sufficiently challenging, I'm sure. Once they are installed, chess.el
will use them, provided the locations of the binaries is on your PATH.